1. Field of the Invention
This invention relates to a semiconductor wafer breaking apparatus for dividing a semiconductor wafer into a large number of chips and particularly it relates to an apparatus for dividing into a large number of chips a semi-full cut wafer put on an adhesive sheet.
2. Description of the Prior Art
As is well known, the operation of making cuts in a semiconductor wafer, with portions of a very small thickness being left uncut, along the dicing lines for separating a large number of devices formed on the upper surface of the semiconductor wafer to provide single devices is generally called semi-full cutting operation. This operation has been popularly done in manufacturing of semiconductor devices. In the following, this operation will be referred to simply as semi-full cutting operation.
FIGS. 1 to 3 are views for explaining the semi-full cutting operation of a wafer. Particularly, FIG. 1 is a perspective view showing a state in which a wafer is set; FIG. 2 is a sectional view of the wafer, the adhesive sheet and the frame in FIG. 1, taken along the line A--A; and FIG. 3 is an enlarged view of the region B shown in FIG. 2.
Referring to FIGS. 1 to 3, the semi-full cutting operation of the wafer will be described in the following.
First, the wafer 3 is put on the adhesive surface of an adhesive sheet 2 uniformly provided on a ring-shaped frame (referred to hereinafter simply as the frame 1) as shown in FIG. 1. Then, the frame 1 is positioned appropriately and the above described semi-full cutting operation is performed by using a cutter generally called a dicer. The wafer thus semi-full cut is shown in FIG. 3. As shown in FIG. 3, notches 31 are formed in the wafer 3 at predetermined intervals.
The semi-full cut wafer 3 is divided into chips so as to undergo a subsequent process called die bonding. In the die bonding, the chips are picked up one by one so that they are bonded.
FIG. 4 is a perspective view of a conventional semiconductor wafer breaking apparatus. Now referring to FIGS. 1 to 4, conventional wafer dividing operation using the apparatus in FIG. 4 will be described sequentially in detail.
Referring to FIG. 4, notches 31 as shown in FIGS. 2 and 3 are generally provided in the wafer put on the adhesive surface of the adhesive sheet 2 uniformly attached to the frame 1. The amount left uncut is generally about 20 to 30 .mu.m. Subsequently, as shown in FIG. 4, the semi-full cut wafer 3 is positioned by using a table 5 which comprises a frame positioning and fixing mechanism 4 and is movable along two axes perpendicular to each other. Then, the semi-full cut wafer 3 together with the adhesive sheet 2 is pushed upward by a prescribed amount using a breaking mechanism 8 under the lower surface of the adhesive sheet 2, located in the lower portion of the frame positioning and fixing mechanism 4. The breaking mechanism 8 comprises a break pin 6 having a hemispheric edge portion with a radius of R, and a vertical movement mechanism 7 for raising and lowering the break pin 6. Then, the semiconductor wafer 3 thus pushed upward by the breaking mechanism 8 from the back surface of the adhesive sheet 2 is moved on the breaking mechanism 8 by the table 5 along the dicing lines read in advance by identifying means (not shown). As a result, the wafer 3 is divided into chips.
Such wafer dividing operation will be described in more detail referring to FIGS. 5 to 7.
FIGS. 5 and 6 are sectional views of the semiconductor wafer breaking apparatus shown in FIG. 4 taken along the line D--D. These figures illustrate a breaking operation of the break pin 6 wih respect to the semi-full cut wafer 3 positioned together with the frame 1 by the positioning and fixing mechanism 4.
The break pin 6 located under the back surface of the adhesive sheet 2 as shown in FIG. 5 is raised by the vertical movement mechanism 7 by a prescribed amount as shown in FIG. 6. Subsequently, the semi-full cut wafer 3 is moved by the above stated table 5 along the dicing lines with the wafer 3 being pushed upward by the break pin 6 from the back surface of the adhesive sheet 2.
Then, in the state in which the adhesive sheet 2 is pushed upward by the break pin 6, as shown in FIG. 7 illustrating an enlarged view of the portion E in FIG. 6, a portion 3' not divided out of the wafer 3 is supported by a supporting point O which is a contact point between the break pin 6 and the adhesive sheet 2. Tension T is applied to an end G of the portion 3'. Though not shown, tension is also applied to the whole area of the portion 3'. In other words, bending stress and tensile stress are generally produced in the non-divided portion 3' of the wafer 3. The stresses are particularly concentrated on the portion F left uncut and accordingly if stronger force than the strength of the material of this portion F is applied to this portion F, the wafer 3 is broken at this portion F.
From the foregoing description, in order to facilitate the division of the wafer, some approaches may be considered. For example, decrease of the strength of the material of the remaining portion F, or decrease of the distance L2 from the supporting point O to the remaining portion F to concentrate stress, or increase of the breaiing force may be considered as those approaches.
In order to decrease the strength of the material of the remaining portion F to a minimum value or 0, a method of cutting the wafer 3 entirely by dicing applied as far as a part of the surface of the adhesive sheet 2 may be considered. However, this full cutting method is not practical because it has a drawback that the adhesive agent of the adhesive sheet 2 is stuck to the cutting edges of the dicer to decrease considerably the lifetime of the cutting edges. As an approach for decreasing the distance L2, a method of decreasing, to the minimum, the radius R of curvature of the hemispheric edge portion of the break pin 6 may be considered. However, in this method, if the wafer 3 is moved by the table 5 so as to be broken along the dicing lines, a so-called stick-slip phenomenon occurs between the break pin 6 and the adhesive sheet 2, causing damage to the back surface of the adhesive sheet 2. Therefore, this method cannot be used practically. It was made clear experimentally that a stick-slip phenomenon can be observed if the radius of curvature of the edge portion of the break pin 6 becomes about 1 mm or less.
In the following, let us consider a method of increasing the breaking force, particularly a bending moment of [Tz.multidot.L1] where Tz is a vertical component of the tension T and L1 is a distance between the acting point G of the vertical component Tz and the supporting point O. Increase of the vertical component Tz can be attained by increase of the raised amount of the break pin 6. However, if the raised amount is increased, the back surface of the adhesive sheet 2 might be damaged or transformed. It was made clear experimentally that it is necessary to limit the raised amount to less than about 5 mm.
The distance L1 varies dependent on the pattern of a device formed on the wafer 3 and the chip size. Particularly, in the case of a small chip of 3 mm.sup.2 or less, the bending moment obtained is extremely small and breaking errors often occur when the wafer is divided by using the break pin 6.
In addition, because of the hemispheric shape of the edge portion of the break pin 6, there are involved serious disadvantages that if the wafer 3 is moved by the table 5 along the dicing lines, cracking occurs in portions other than the dicing lines or complicated cleavage faces are formed along the breaking lines because the breaking force is also applied in the directions other than the moving directions, and that a large amount of dust of the wafer (silicon dust) is produced and scattered on the upper surface of the device region to cause damage to the device.
Techniques of dividing a semiconductor wafer are disclosed for example in Japanese Patent Laying-Open Gazette Nos. 90478/1973 and 73871/1976. However, any of them cannot solve the above described problems.